In the present invention, the terms “silicone rubber sponge composition” and “silicone rubber sponge (cured product)” imply respectively a cellular rubber composition having interstices originating from an organic resin hollow filler and a cured product of the cellular rubber composition. They may occasionally be replaced by the synonymous terms “silicone rubber foam composition” and “silicone rubber foam,” respectively.
Liquid silicone rubber compositions of heat-curable type find use in a variety of fields because of their good moldability and their ability to form, cured products (silicone rubbers) after molding having excellent heat resistance and electrical insulating properties. Among such compositions are those having a low specific gravity which are required in many fields on account of their possible contribution to size and weight reduction. They are widely used for the toner fixing roll of the electrophotographic image forming device such as PPC, LBP, and FAX because of their excellent heat resistance and mold release characteristics. The devices that employ the electrophotographic process need a step of fixing the toner image to copy paper after its transfer from the surface of the photosensitive body to copy paper. Usually, the fixing of toner images takes place as copy paper passes through the gap between a heating roll and a pressing roll which rotate pushing each other. This rolling step causes the toner to fix onto copy paper by heat fusion. The heat fusion method permits the copying machine or printer to respond rapidly if the roll is made of a material with high thermal conductivity. Unfortunately, such a material is liable to rapid heat dissipation which is not desirable for size reduction and cost saving. This has stimulated a new demand for a material having a low thermal conductivity or excelling in heat storing characteristics. Such a material is disclosed in JP-A H05-209080 (Patent Document 1). It contains a solid filler of organic resin which expands upon heating. Unfortunately, it involves difficulties in molding (such as lack of foam uniformity) because it relies on in-mold expansion. There is disclosed a liquid silicone rubber composition for incorporated with hollow (expanded) plastic particles, in JP-A H09-137063 (Patent Document 2). Patent Document 2 mentions that the composition may contain an optional inorganic filler such as fumed silica, precipitated silica, quartz powder, and diatomaceous earth. However, Document 2 mentions nothing about the filler for its amount, thermal conductivity, and heat capacity. In fact, it merely gives an example that treats vapor phase method (fumed silica). Fumed silica and precipitated silica are not desirable fillers because they tend to produce an adverse effect on permanent compression set although they can impart strength to silicone rubber. This holds true particularly in the case of gas-containing low-specific gravity silicone rubber (in sponge form). Among other low-specific gravity materials for the toner fixing roll are those incorporated with an expanded hollow filler, which are disclosed in Japanese Patent Nos. 3274071 and 3494039 (Patent Documents 3 and 4). The former only discloses carbon black as an inorganic filler, and the latter only discloses (without details) silica finer powder, calcium carbonate, or iron oxide as an optional component (with fumed silica alone used in Examples). There is disclosed in JP-A 2001-220510 (Patent Document 5) a material superior in permanent compression set which is incorporated with an expanded organic resin filler in combination with a polyhydric alcohol. This disclosure merely mentions the inorganic filler as an optional component and gives Examples which employ fumed silica alone.
The recent requirement for printers with a higher throughput has led to an instance in which sufficient heating is not achieved by the toner melting mechanism alone (such as heater-containing roll or belt, ceramic heater, and IH heater).
The above-mentioned sponge roll of low thermal conductivity type, which is used as a “pressing sponge roll” to be pressed against the toner melting mechanism, suffers the disadvantage of rapidly decreasing in the surface temperature because of its low thermal conductivity once it has its heat taken away from its printing surface in contact with copy paper. This disadvantage leads to the change in roll diameter or the roll lacking uniform outside diameter.
Although the foregoing is not the case with a pressing roll of unexpanded solid rubber with high thermal conductivity (which has no hollow parts unlike sponge), the pressing roll of solid rubber suffers the disadvantage of requiring a large amount of thermal energy to heat it entirely to the toner melting temperature. The result is an extended warm-up time for print starting. Therefore, the pressing roll should ideally be made of a rubber having a high thermal conductivity and a small heat capacity.